The present invention relates to a polishing apparatus and a polishing method for polishing a semiconductor wafer flat like a mirror, particularly to a polishing apparatus.
In recent years, as the integration density of a semiconductor wafer is increased, circuit wires become finer, the distance between wires becomes smaller, and the number of wiring layers increases. For this reason, the semiconductor wafer and wiring layers are required to be flat. To efficiently flatten semiconductor wafers, the CMP (chemical mechanical polishing) method is applied.
FIG. 4 shows a schematic diagram showing a polishing apparatus used to perform polishing by the CMP method.
A polishing apparatus 1 comprises a carrier 3 for holding a work 2 such as a semiconductor wafer, a rotation motor (not shown) for rotationally driving the carrier 3, an air cylinder 6 for pressing the carrier 3 and a drive motor 4 against a table 5 (referred to later), the table 5 opposite to the carrier 3, a polishing cloth 7 as a polisher made of nonwoven fabric or expanded polyurethane and provided on the table 5, and the drive motor 4 for rotationally driving the table 5.
A polishing solution containing suitable abrasive grains and chemical liquid is supplied to a portion between the polishing cloth 7 and the work 2, so that the work 2 can be polished to a desired work accuracy.
The work 2 is held by that surface of the carrier 3 opposite to the upper surface of the table 5. The periphery of the work 2 is held by a guide ring to prevent the work 2 from moving off the carrier 3 during the polishing process.
The work 2 can be held by the carrier 3 by various methods. For example, the work 2 can be adhered to the carrier 3 by vacuum; adhered by wax applied to the carrier 3; or fixed by water on a soft film provided on the carrier 3. In the state where the work 2 is adhered to the carrier 3, the carrier 3 and the table 5 are rotated by the drive motor 4 and pressing force is applied to the work 2 by the air cylinder 6. As a result, the irregularities on a surface of the work 2 to be polished can be removed by both the physical function of the grains and the chemical function of the liquid contained in the polishing solution, thereby finishing the surface to a flat face.
If there is no change in polishing environment of the polishing cloth 7 or the polishing solution, the amount of removed part of the work can be determined by the polishing pressure, the relative speed between the work 2 and the polishing cloth 7 and the period of polishing time. Therefore, if the work 2 has a particularly rugged portion, it is necessary to increase the pressure applied to the rugged portion, the relative speed between the work 2 and the polishing cloth 7, or the polishing time.
However, the polishing environment of the polishing cloth 7 or the polishing solution is not constant. In particular, the polishing cloth 7 is worn in accordance with an increase in the period of polishing time and the number of times of polishing. This changes the characteristics of the polishing cloth 7 or the polishing solution, such as the elasticity coefficient and the ability of maintaining polishing grains, which influence the polishing and removing rate and the pressure. Therefore, in the polishing apparatus 1, the most worn portion of the polishing cloth 7 is on a rotation trail in a central portion of the work 2, where the polishing period of time per unit area is the longest. For this reason, if the carrier has the only one shape determined under specific conditions, once the polishing environment is changed, the work 2 cannot be polished precisely flat.
To solve this problem, various methods have been proposed. In one method, the flatness of the work is monitored while polished, and the polishing conditions are changed in accordance with a change in polishing environment during the polishing process. In another method, the shape of the carrier 3 is changed during the polishing process in accordance with the polishing state, so as to adjust the dimensions of the carrier 3.
However, even when the polishing conditions or the shape of the carrier 3 is changed in accordance with the change in polishing environment as described above, a turned-down edge (a phenomenon in which an edge portion of the work 2 is thinned) may occur, resulting in reduction of the flatness of the work 2.
As described above, the problem in an inner portion of the work 2, other than the edge portion, is overcome by optimizing the shape of the carrier to change the pressing force applied to the work 2. However, since the aforementioned turned-down edge results from a problem of the pressure distribution in a peripheral portion of the work 2, it cannot be overcome only by optimizing the shape of the carrier.
FIG. 5 schematically shows a status of a pressure generated in a peripheral portion of the work 2. In this portion, a synthetic pressure is generated by synthesis of a pressing force generated by pressing the work 2 against the polishing cloth and a pressure (hereinafter referred to as a dynamic pressure) generated by relative movement between the work 2 and the polishing cloth 7 (movement obtained by synthesizing rotation of the table 5 and the rotation of the work 2 held by the polishing apparatus 1).
For this reason, the pressure in the peripheral portion of the work 2 is adjusted by the guide ring 8 provided to prevent the work 2 from moving. To suppress the dynamic pressure, it is only necessary to optimize the height of the guide ring 8. However, due to a change in polishing environment, as in the case of changing the shape of the carrier, the optimal effect cannot be maintained only by adjusting the height of the guide ring 8.
To solve this problem, according to the method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-168964, a press load of the guide ring 8 on the polishing cloth 7 can be varied independent of that of the carrier 3, so that the press load of the guide 8 can be greater than that of the carrier 3.
However, when the press load of the guide ring 8 is increased, the amount of the polishing solution supplied to that portion of the work 2 under polishing is reduced, since the supply of the solution is cutoff by the guide ring 8. As a result, the polishing rate is lowered, inevitably reducing the production yield of works 2.
Further, Jpn. Pat. Appln. KOKAI Publication No. 8-11055 discloses a structure in which a guide ring 8 has a number of grooves arranged at intervals to satisfactorily supply the polishing liquid to a work 2 held by the carrier 3. However, according to the technique disclosed in this reference, a holding portion holding a work 2 is formed integral with the guide ring 8. Hence, the position of the grooves relative to the work 2 cannot be changed. For this reason, in a peripheral portion of the work 2, the pressing force varies depending on ruggedness due to the grooves, and influences the work 2. Therefore, the work 2 cannot be polished flat in the peripheral portion.